1. Field of the Invention:
The present invention relates to the treatment of magnesium alloys with a view to improving their corrosion resistance.
2. Description of the Related Art:
Magnesium alloys are of great benefit owing to their lightness. In particular, they are applicable in the transport (automobile and aeronautical) industries, medical equipment and mobile telephony. One of the weak points of these materials is their sensitivity to corrosion. Corrosion protection may be obtained by depositing a coating or by treating the surface. The protective layers may be produced either by chemical conversion or by anodizing in solutions containing metal salts and metalloids.
Anodizing treatments have been carried out in electrochemical cells in which the electrolyte contains a peroxide or a powerful oxidizing agent such as a chromate, a vanadate or a permanganate. One drawback of using such electrolytes lies in the fact that the transition metal ions are present, including some that are in the form of toxic species, for example Cr6+. Other electrolytes not containing transition metals have been proposed. Thus, U.S. Pat. No. 4,978,432 describes the anodizing of parts made of a magnesium alloy using an electrolyte containing borate and/or sulfate anions introduced in acid form. However, the protective layer that forms on the surface of the magnesium alloy part has a high porosity, which impairs the effectiveness of the protection.